An exhaust pipe of an internal combustion engine is provided with an exhaust gas purifying apparatus such as a catalyst for purifying exhaust gas. It is known that a secondary air supply apparatus such as an air pump is provided upstream of the exhaust gas purifying apparatus in order to supply secondary air into the exhaust pipe.
A DC motor, for example, drives the air pump. A temperature of the whole pump, which is referred to as a pump temperature hereinafter, increases according as an operation period of the pump elapses. Thus, a long period operation of the air pump excessively increases the pump temperature, so that the air pump may receive damages and/or a lifetime of the air pump is reduced. In order to overcome these problems, the operation period of the air pump is predetermined and the air pump is operated within the predetermined period.
In a secondary air supply apparatus shown in JP-2000-240434A, it is determined whether a non-operating period of the internal combustion engine is short based on a difference in a temperature of coolant between at a time when the internal combustion engine is stopping and a time when the internal combustion engine is restarting. When it is determined the non-operating period is short, the operation period of the air pump is corrected according to a difference in temperature between coolant and an intake air at the time when the internal combustion engine is restarted.
However, in the above prior art, although the pump operating period is determined based on the temperature conditions before the air pump is started to be driven, the air pump is submissively driven until the predetermine period has elapsed. Thus, even if an operating condition of the air pump is varied during the operating period, the operating period cannot be adjusted. When the operation condition of the air pump is changed into a high load condition, a load of the air pump is increased compared with a normal load operation, so that damages to the air pump and/or a reduction of lifetime of the air pump are arouse.